It is conventional to produce pentachloro-3-butenoic acid esters by adding two chlorine atoms to 1-ethoxypentachloro-1,3-butadiene; splitting off chloroethane from the thus-formed 1-ethoxyheptachloro-3-butene by heating; and reacting the thus obtained pentachloro-3-butenoic acid chloride with an alcohol [Liebigs Ann. Chem. 600:1 (1956)]. However, this process has considerable disadvantages. For instance, when producing the starting material 1-ethoxypentachloro-1,3-butadiene from hexachloro-1,3-butadiene (conversion about 60%, selectivity about 65%), by-products--in most cases higher-boiling compounds--are produced to a considerable extent. On the one hand, in attempting to separate these by-products and the unreacted hexachloro-1,3-butadiene by distillation on an industrial scale (in the presence of iron-containing vessel materials), a considerable portion of the 1-ethoxypentachloro-1,3-butadiene is decomposed. On the other hand, if this separation is not performed and the step of splitting off chloroethane from the 1-ethoxyheptachloro-3-butene is conducted using the crude product still containing hexachloro-1,3-butadiene and by-products, the yield of pentachloro-3-butenoic acid chloride is drastically reduced (see Comparative Example A).
It is furthermore known to prepare pentachloro-3-butenoic acid esters by adding two chlorine atoms to tetrachlorocyclobutenone and reacting the thus-produced pentachloro-3-butenoic acid chloride with an alcohol (DOS [German Unexamined Laid-Open Application] No. 2,754,670 and corresponding to U.S. Pat. No. 4,175,095). This process, however, has attendant problems with regard to the preparation of the starting compound, using tetrachlorocyclobutenone.
This is evidenced by the results of the attempt to split off chloroethane from 1-ethoxypentachloro-1,3-butadiene to obtain tetrachlorocyclobutenone (German Pat. Nos. 1,206,433 and 1,271,111) on an industrial scale. Although the reaction takes place satisfactorily on a laboratory scale, only small yields are produced. For another tetrachlorocyclobutenone preparation, the starting compound, 3, 3, 4, 4,-tetrafluoro-2-chloro-1-methoxy-1cyclobutene (Chem. Ber. [Chemical Reports] 99:1966 [1970]), is accessible only with difficulty. Furthermore, tetrachlorocyclobutenone can also be obtained in a smooth reaction from hexachlorocyclobutene according to a process not yet part of the state of the art (German patent application P No. 28 10 398.2 and corresponding to U.S. application Ser. No. 019,105 filed on Mar. 9, 1979); however, the production of the latter compound is burdened by high initial investment costs occasioned by a requisite very expensive distillation.
As can be seen, there is presently no fully satisfactory process for preparing pentachloro-3-butenoic acid esters.